body {
  background-image: url(http://www.hdwallpapers.in/walls/star_trail-wide.jpg);
  background-size: cover;
  background-repeat: no-repeat;
  background-attach: fixed;
  animation: animate-bg 50s infinite linear;
}

@keyframes animate-bg {
  from { background-size: 100%; background-position: 0% 0%; }
  50% { background-size: 120%; background-position: 0% 20% }
  to { background-size: 100%; background-position: 0% 0%; }
}

External CSS

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External JavaScript

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